Magnetoresistive random-access memory (“MRAM”) is a non-volatile memory technology that stores data through magnetic storage elements. In a type of MRAM, the magnetic storage elements comprise two ferromagnetic plates or electrodes that can hold a magnetic field and are separated by a non-magnetic material, such as a non-magnetic metal or insulator. Such a structure is called a magnetic tunnel junction (“MTJ”). In general, one of the plates has its magnetization pinned (i.e., a “reference layer”), meaning that this layer has a higher coercivity than the other layer and requires a larger magnetic field or spin-polarized current to change the orientation of its magnetization. The second plate is typically referred to as the free layer and its magnetization direction can be changed by a smaller magnetic field or spin-polarized current relative to the reference layer. Thus, the free layer is also referred to as the storage layer. MTJ's are manufactured using stacked materials, with each stack of materials forming an MTJ pillar.
MRAM devices store information by changing the orientation of the magnetization of the free layer. In particular, based on whether the free layer is in a parallel or anti-parallel alignment relative to the reference layer, either a “1” or a “0” can be stored in each MRAM cell. Due to the spin-polarized tunneling magnetoresistance (TMR) effect, the electrical resistance of the cell change due to the orientation of the magnetic fields of the two layers. The cell's resistance will be different for the parallel and anti-parallel states and thus the cell's resistance can be used to distinguish between a “1” and a “0”. One important feature of MRAM devices is that they are non-volatile memory devices, since they maintain the information even when the power is off. The two plates can be sub-micron in lateral size and the magnetization direction can still be stable with respect to thermal fluctuations.
Spin transfer torque or spin transfer switching, uses spin-aligned (“polarized”) electrons to change the magnetization orientation of the free layer in the magnetic tunnel junction. In general, electrons possess a spin, a quantized number of angular momentum intrinsic to the electron. An electrical current is generally unpolarized, i.e., it consists of 50% spin up and 50% spin down electrons. Passing a current though a magnetic layer polarizes electrons with the spin orientation corresponding to the magnetization direction of the magnetic layer (i.e., polarizer), thus produces a spin-polarized current. If a spin-polarized current is passed to the magnetic region of a free layer in the magnetic tunnel junction device, the electrons will transfer a portion of their spin-angular momentum to the magnetization layer to produce a torque on the magnetization of the free layer. Thus, this spin transfer torque can switch the magnetization of the free layer, which, in effect, writes either a “1” or a “0” based on whether the free layer is in the parallel or anti-parallel states relative to the reference layer.
MRAM devices are considered as the next generation structures for wide range of memory applications. One MRAM technology uses a perpendicular magnetic tunnel junction device. In perpendicular MTJ devices, the free and reference layers are separated by a thin insulator layer for spin polarized tunneling. The free and reference layers have a magnetic direction that is perpendicular to their planes, thus creating a perpendicular magnetic tunnel junction (pMTJ). The pMTJ configuration may provide a lower critical switching current when compared to in-plane MTJ technology, simplified layer stack structure without need of using thick antiferromagnetic layers, and reduction of the device size below 40 nm.
FIG. 1 illustrates a pMTJ stack 100 for a conventional MRAM device. As shown, stack 100 includes one or more seed layers 110 provided at the bottom of stack 100 to initiate a desired crystalline growth in the above-deposited layers. A perpendicular synthetic antiferromagnetic layer (“pSAF layer”) 120 is disposed on top of the seed layers 110. MTJ 130 is deposited on top of synthetic antiferromagnetic (SAF) layer 120. MTJ 130 includes reference layer 132, which is a magnetic layer, a non-magnetic tunneling barrier layer (i.e., the insulator) 134, and the free layer 136, which is also a magnetic layer. It should be understood that reference layer 132 is actually part of SAF layer 120, but forms one of the ferromagnetic plates of MTJ 130 when the non-magnetic tunneling barrier layer 134 and free layer 136 are formed on reference layer 132. As shown in FIG. 1, magnetic reference layer 132 has a magnetization direction perpendicular to its plane. As also seen in FIG. 1, free layer 136 also has a magnetization direction perpendicular to its plane, but its direction can vary by 180 degrees.
The first magnetic layer 114 in the perpendicular SAF layer 120 is disposed over seed layer 110. Perpendicular SAF layer 120 also has an antiferromagnetic coupling layer 116 disposed over the first magnetic layer 114. As seen by the arrows in magnetic layers 114 and 132 of perpendicular SAF 120, layers 114 and 132 have a magnetic direction that is perpendicular to their respective planes. Furthermore, a nonmagnetic spacer 140 is disposed on top of MTJ 130 and a polarizer 150 may optionally be disposed on top of the nonmagnetic spacer 140. Polarizer 150 is a magnetic layer that has a magnetic direction in its plane, but is orthogonal to the magnetic direction of the reference layer 132 and free layer 136. Polarizer 150 is provided to polarize a current of electrons (“spin-aligned electrons”) applied to pMTJ structure 100. Further, one or more capping layers 160 can be provided on top of polarizer 150 to protect the layers below on MTJ stack 100. Finally, a hard mask 170 is deposited over capping layers 160 and is provided to pattern the underlying layers of the MTJ structure 100, using a reactive ion etch (RIE) process.
As discussed, one type of MTJ is referred to as a perpendicular MTJ. In a perpendicular MTJ, the reference layer and the free layer each have a magnetic direction that is perpendicular to the plane of their respective layers. The resistance of the magnetic memory device is sensitive to the relative orientation of the magnetization vector of the free magnetic layer and the magnetization vector of the reference layer. The resistance of the magnetic memory device is highest when the magnetization vectors of the free magnetic layer and the reference layer, respectively, are in anti-parallel alignment. The resistance of the magnetic device is lowest when the magnetization vectors of the layers free magnetic layer and the reference layer, respectively, are in parallel alignment. Thus, a resistance measurement or its equivalent can determine the orientation of the magnetization vector of the free magnetic layer.
An important characteristic of MTJs is thermal stability. The thermal stability of each perpendicular MTJ, i.e., the magnetic bits, is proportional to the magnetic material volume of the MTJ for a given perpendicular anisotropy. Thermal stability of an MTJ is a factor in data retention capability. Thus, improving the thermal stability of the free layer of an MTJ is an important design consideration. Because of the relationship between the magnetic material volume of an MTJ and the perpendicular anisotropy, as MTJ pillar dimensions decreases, for example when shrinking an existing design for future generation MRAM devices, the thermal stability declines. This is highly undesirable. Unfortunately the thickness of the free layer cannot be increased at will to add more magnetic moment (volume) to enhance the thermal stability. Thus, the thermal stability of the free layer structure with a perpendicular magnetic direction cannot be enhanced simply by increasing the thickness of the material used to construct the free layer (typically CoFeB). This is because there is a limit on the thickness of CoFeB where the perpendicular anisotropy can be obtained. For CoFeB, this thickness may be around sixteen (16) Angstroms. Above this thickness, the magnetization reverses to be in plane, meaning that the MTJ will no longer be a perpendicular MTJ. Thus, the thermal stability of the perpendicular MTJ free (i.e., storage) layer cannot be enhanced by further increasing the free layer thickness.
Perpendicular magnetization direction can be achieved using surface perpendicular anisotropy (interface perpendicular magnetic anisotropy) which is an interface property of the ferromagnetic film and neighboring capping and seeding layer of non-magnetic material used for a free layer. Interface perpendicular magnetic anisotropy (IPMA) is inversely proportional to the thickness of the film. For common ferromagnetic materials, IPMA becomes strong enough to keep magnetization out of plane in the thickness range of 1.2 to 1.6 nm. However, at this thicknesses range, the magnetic moment of the free layer is small. This small magnetic moment of the free layer reduces thermal stability. On the other hand, increasing the free layer thickness lowers the IPMA, which causes the free layer to become in-plane magnetized. In a perpendicular MTJ device, this is not acceptable since it would cause degradation of the tunneling magnetoresistance (TMR) value to a level below which device can operate reliably. Thus, increasing the free layer thickness lowers the thermal stability by diminishing the perpendicular anisotropy. In addition, the device itself becomes useless, as the free layer loses its perpendicular magnetic anisotropy. This is one of the most difficult issues to address for perpendicular MTJ MRAM devices.
Thus, a need exists to enhance the thermal stability of the free layer of an MTJ where the thickness of the free layer does not have to be disturbed.